Field of the Invention
The present invention relates to an image pickup apparatus, such as a digital camera, and more particularly to a technique for correcting exposure unevenness due to flicker (i.e. change in ambient light which occurs during photographing).
Description of the Related Art
In many image pickup apparatuses, such as digital cameras, with a recent increase in the ISO sensitivity, it has become possible to perform high-speed shutter photographing even under an artificial light source which causes flicker. High-speed shutter photographing is advantageous in that a photograph without image blur can be obtained e.g. in photographing of an indoor-sports scene. However, in high-speed shutter photographing under a flicker light source, frame-by-frame variation in image exposure or color temperature can occur due to the influence of flicker.
To cope with the above-mentioned problem, there has been proposed a technique in which flicker is detected and then exposure is performed in timing corresponding to the peak position of the flicker, where change in brightness is smallest, so as to reduce the influence of the flicker (see Japanese Patent Laid-Open Publication No. H06-209427).
Further, there has been proposed a technique in which after images are sequentially picked up in predetermined timing, signal levels of the respective images are determined, and an image of a reference signal level is determined based on a result of the determination, whereby the signal levels of the respective images are corrected according to a ratio between the signal level of the determined reference image and that of each of the other images (see Japanese Patent Laid-Open Publication No. 2008-199144). According to this proposal, when the ratio exceeds a predetermined range, only the reference image is output.
However, the technique proposed in Japanese Patent Laid-Open Publication No. H06-209427 has a problem that in still image photographing, when exposure is performed in timing corresponding to a flicker peak position, a release time lag occurs depending on the flicker timing. Accordingly, in still image continuous photographing, it is required to await the peak position of the flicker for each frame and then release a shutter, and therefore a time period taken to complete one-frame exposure can be increased depending on flicker timing, which can lead to reduction of frame speed. Further, in the case of Japanese Patent Laid-Open Publication No. 2008-199144, since the signal level of each image is corrected by multiplying the signal level by a gain, noise can be generated in the image, causing degradation of image quality.